1. Field of the Invention
The present invention relates to an incombustible honeycomb radio absorptive material and to a radio absorber using it. More specifically, the invention relates to an incombustible honeycomb radio absorptive material which is suitably used for an anechoic chamber (radio shielding room), has a high incombustibility, is excellent in radio absorptivity, is lightweight, and has a high strength, as well as to a radio absorber using the material.
2. Description of the Related Art
Recently, for an advanced information society, the utilization of radio waves has been rapidly expanded particularly in the mobile communication field. Also, with today's innovative progress of microelectronics techniques, various kinds of electronic instruments have come into widespread use. Under these circumstances, a hindrance occurred by unnecessary electromagnetic waves, such as electromagnetic wave noises, from those electronic instruments has become a large problem.
The measurement of said electromagnetic wave noises is usually carried out in an anechoic chamber where no reflection is caused of electromagnetic waves, and the inside wall of the anechoic chamber is made with a radio absorber. For the radio absorber, it is mostly used an organic material, such as foamed polystyrene and foamed polyurethane, where each being compounded with carbon black for improving electric conductivity, however, incombustibility is low of foamed polystyrene or foamed polyurethane. As radio interference has increased recently, the test of a large electric power, such as an immunity test, would be a mandatory one in Japan. As the anechonic chamber requires works in a closed space, a radio absorber to be used for said chamber is required to have, in addition to radio absorptivity, a higher incombustibility for improving safety.
Radio absorbers having incombustibility have been proposed as follows: a heat-resisting incombustible radio absorber made of a shaped material formed by cumulating and bonding many closed cellular inorganic particles with a heat-resisting inorganic adhesive having a definite amount of carbon black dispersed therein (Japanese Patent No. 2,743,227); a radio absorber obtained by attaching and fixing an electrically conductive material to a molded material comprising ceramic or glass short fibers each having a specific aspect ratio by penetration and impregnation (Japanese Patent Laid-Open No. 307,268/1997); and a radio absorber made of a composition constituted by a cement, a light-weight aggregate, non-electrically conductive fibers, and a synthetic resin emulsion (Japanese Patent Laid-Open No. 67,544/1996).
However, the heat-resisting incombustible radio absorber of Japanese Patent No. 2,743,227 is structurally weak, having low mechanical strength, and as it is formed with many closed cellular particles, dusts are apt to be generated during assembling the absorber and by a mutual-contact of the particles.
In the radio absorber proposed in Japanese Patent Laid-Open No. 307268/1997, a manufacturing cost is very high.
In the radio absorber proposed in Japanese Patent Laid-Open No. 67,544/1996, large amounts of organic hollow particles and an organic binder are used to reduce a weight of a mortal be lightweight, however, as they are quasi-incombustible materials or smoke-producing materials, fuming amounts from them are greatly large compared to those from incombustible materials.
Furthermore, the above conventional radio absorbers each is heavy in its own weight, and therefore, each of them is easily distorted by its own weight to cause deformation or breakage.
In order to prevent the deformation or breakage, several techniques have been proposed so that radio absorbers be made in honeycomb structures (Japanese Patent Laid-Open Nos. 205,000/1991 and 132,691/1994, etc.). However, those radio absorbers are quite inferior in fire resistance, as the supporting materials therefor are papers or plastics.